Incidence Rate of Adult ADHD Growing in Japan

Newly approved treatments and a general increase in awareness could help explain why incidence rates of attention deficit/hyperactivity disorder (ADHD) is increasing among adults in Japan.

A team, led by Daimei Sasayama, MD, PhD, Department of Psychiatry, Shinshu University School of Medicine, examined trends in the nationwide incidence of newly diagnosed ADHD between April 2010 and March 2020.

While ADHD is extremely common in childhood, about 50% of pediatric cases will persist into adulthood.

“However, adult ADHD is often underdiagnosed owing to inadequate recognition of the disorder,” the authors wrote.

Checking Healthcare Data on ADHD

In the study, the investigators used the National Database of Health Insurance Claims and Specific Health Checkups of Japan (NDB) and collected demographic and diagnostic information, including all electronic health insurance claims between April 2009 and March 2020.

Overall there were 838,265 patients diagnosed with ADHD between the fiscal year 2010 and 2019, including 121,278 patients diagnosed aged 0-6 years and 381,753 patients diagnosed with ADHD aged 7-19 years.

Overall, during the study period, the incidence of newly diagnosed ADHD increased 2.7 fold in the 0-6 age group, 2.5 fold in the 7-19 age group, and 21.1 fold among adults older than 19 years.

Potential Reasonings

The authors offered reasons for the increase in ADHD diagnoses.

“One reason may be the release of the Diagnostic and Statistical Manual of Mental Disorders (Fifth Edition) (DSM-5) in 2013, which allows combined diagnosis of ADHD and autism spectrum disorder (ASD),” the authors wrote. “Although the ICD-10 precludes a dual diagnosis of ADHD and ASD, the release of the DSM-5 may have prompted physicians to give an additional diagnosis of ADHD to individuals with ASD to prescribe ADHD medications.”

The results also show a bigger increase among adults between 2012-2017, which could be due to increased sensitivity of diagnosis, particularly of adults with ADHD.

In 2012, the Japanese regulatory body approved the first medication for adults with ADHD, which could explain the increased ADHD awareness among this patient population.

The annual incidence of adult ADHD seems to have leveled after reaching a peak of 6 per 10,000 person-years in 2018, which is comparable to the adult ADHD incidence of Asian individuals in the US in 2016.

“Thus, the sensitivity to detect ADHD in the adult population seems to have reached a standard in Japan,” the authors wrote.

Limitations and the Need for Future Research

There were some limitations of the study, including that the available demographic data was restricted to just sex and age groups.

However, the nonetiological factors could help explain partially the apparent increase in ADHD incidence and further research is suggested to show how etiological and nonetiological factors impact the changing incidence of ADHD in Japan.

The study, “Trends in Diagnosed Attention-Deficit/Hyperactivity Disorder Among Children, Adolescents, and Adults in Japan From April 2010 to March 2020,” was published online in JAMA Network Open.